Leacture three limb bud formation Flashcards
what are the three parts of a limb (p to D)
stylopod, zeygopod, autopod
when does limb foration start in the chick
3 days
when does kimb formation start in the mouse and when is it formed
10 daysto 14 days
where doe the limb bud form
junctin between the dorsal and ventral ecotderm
in distinct position along the AP axis
what experiment shows the AP axis is established very early
take presomiic mesoderm and graft back in incerted = somites form in reverse order
this shows cells have been predetermined with their positional identity
how is fgf8 expressed
gradientwith highest at the posterior
what is the expression attern of retinoic acid
is highest in the mid trunk of the embryo and declines both posteriorally and anteriorally
what is the role of the fgf8 and RA gradients
give positional identity to cells along the AP axis
an address of where they are on it
what other signally are also important in establishing the AP axis
WNts and BMPS expressed in posterior (WNT and BMPantagonists expressed in anterior)
what is th eoutcome of the signalling gradients
RA/WNT3a and fgf8 activate TF (cdcxs) these then activate Hox genes
hox genes inv in AP patterning
what are properties of HOx genes
homeotic TFs
evolutionary conserved
show colinearity
in their order along the chromosome and spatial/temporal pattern along the AP axis
where are 3’ hox genes expressed
at the anterior
what do the expression pattern of Hoxb genes show
shows anterior borders are distinct whilst posterior are not
how does hox expression explain the difference in the number of cervical vertebrae on the mouse
the chick has twice as many as mouse
has a bigger hox 5 expression (=hox5/6 boundary is change in the type of rib)
what does hox expression in the snake show
doesnt allow for limbs
as norm region anterior to hoxc6 becomes forelimb
in snake hoxc6 expressed thoroughout - no anterior region of its expression = no limb
expansion of hoxc6 and hoc8 expression correlates with the expasion of thoracic identitiy of skeleton as no act of necessary signalling pathways
what is the role of fgf10 and how was this shown
in lpm
initiates limb bud formation
is expressed in the correct areas jus tprior to fomation
ectopic expression induces and ectopic limb
ko = no limb = is necessary
what is the TF that activates fgf10
Tbx5
what controls the position of fgf10
tbx5 activation
tbx5 expression is activated by the rostral hox code (hox4/5)
and tbx5 expression is repressed by caudal hox code expression
where does the fgf10/hox/tbx signalling occur
in the lateral plate mesoderm
what does fgf10 induce in the ectoderm of the limb bud
fgf8 expression
what TF is expressed in the forelimb bud
tbx5
what TF are expressed in the hindlimb bud
Tbx4 and Pitx1
what does inducing an ectopic limb etween the two normal limb buds with a fgf10 bead show
in the ectopic limb there is chimeric expression of the tbx gnees = this is consistent with these genes being progeitors for the HL and FL
in the chick what is the consequence of tbx4 overexpression and what is the consequence of tbx5 overexpression
tbx4 takes on HL identity (wt= FL)
tbx5 takes on FL identity (wt= HL)
what do the overexpression expreiments of tbx genes in the chick show
confirms their role in the FL/HL identitiy
what occurs in the mouse if KO the tbx genes
tbx4 - defect in hl
tbx5 - defect in fl
what do conditional ko and rescue of tbx4 show
(KO tbx5 then put in tbx4) shows that tbx4 is not sufficient to give hindlimb identity in the mouse = ptype still resemble FL
what do experiments with pitx1 show
misexpress in fl gives more hl id
ko = loss of hl feature
when do condtional ko of tbx5 then save with pitx1 = no limb = not sufficient
conditonal ko of tbx5 then rescue with tbx4 and pitx1 = partial hl therefore both imp for fl id
what signalling pathway group do fgf belong to ?
tyrosine receptor kinases
